Primal and subprimal meat cuts are large cuts of meat. They are smaller than a side of beef, for example, but larger than the ultimate cut that is sold at retail to the consumer. A primal cut comprises the entire section of a side of beef, such as the rib section or the rump roast section, while a subprimal cut comprises only a portion of such a section. Primal and subprimal cuts are prepared at the slaughter house and are then shipped to a retail meat store, or to an institution such as a hospital. hotel or restaurant, where they are butchered into smaller cuts of meat suitable for the individual consumer. When primal and subprimal meat cuts, such as roast or rib sections, are prepared at the slaughter house, they are usually packaged in such a way that air (i.e., oxygen) is prevented from contacting the meat and moisture is prevented from leaving the meat. This is done in order to minimize spoilage and discoloration during shipping and handling. One desirable way to package primal and subprimal meat cuts so as to protect them from contact with air and from moisture loss is to shrink package them with a packaging material that has good oxygen and moisture vapor barrier properties. One such shrink packaging material that has good oxygen and moisture vapor barrier properties is polyvinylidene chloride film. (Polyvinylidene chloride copolymers are commonly referred to as PVDC.) However, while polyvinylidene chloride film has excellent barrier properties, in actual practice, when polyviinylidene chloride is used as a monolayer film, it must be plasticized in order for the film to have adequate abrasion resistance and flexibility at storage temperatures of, for example, 30.degree. to 50.degree. F. Unfortunately, the addition of plasticizer sufficient to provide the requisite low temperature properties to the PVDC monolayer film has a significant adverse effect on the barrier properties of the film. While increasing the thickness of the film from the conventional thickness of 1.5 to 2.0 mils, to 5 mils or more, for instance, would improve the barrier properties of the film, it would be economically undesirable to use a monolayer film of polyvinylidene chloride having a thickness of 5 or more mils. Also, if such thick film were employed, bags made from the film would be difficult to gather and clip closed at the open end.
One approach to the provision of a film having barrier properties which are better than those of the 1.5 to 2.0 mil monolayer polyvinylidene chloride film previously used for shrink packaging primal and subprimal cuts is to employ a multilayer film, one layer of which is polyvinylidene chloride having a minimum amount of plasticizer. The other layer or layers of such multilayer films are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in the polyvinylidene chloride layer containing little or no plasticizer. In providing such a film, however, it must be recognized that good oxygen and moisture vapor barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging primal and subprimal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient for the film to heat-shrink within a specified range of percentages, e.g., from about 15 to 60 percent at about 90.degree. C., in both the machine and the transverse directions. (Hereinafter, the term "MD" refers to machine direction and the term "TD" refers to transverse direction.) The film must be heat sealable in order to be able to fabricate bags from the film and in order to heat seal the open ends of the fabricated bags after insertion of the meat product. The heat sealed seams of the bags must not pull apart during the heat shrinking operation, and the film must resist puncturing by sharp bone edges during the heat shrinking operation. Also, there must be adequate adhesion between the several layers of the film so that delamination does not occur, either during the heat shrinking operation or during exposure of the film to the relatively high temperatures that may be reached during shipping and storage of the film in the summertime.
Primal and subprimal meat cuts typically have substantial portions of bones remaining in them, and often the bones will protrude above the surface of the meat itself. This can, and usually does, present a problem when evacuating the meat package since sharp bones can puncture the walls of the bag. A puncture of the bag walls will release the vacuum within the shrink packaging on the meat and will thus cause air to migrate into the bag. The puncture thereby causes deterioration of the meat from contact with the oxygen of the air and from the considerable outward leakage of the moisture contained in the meat.
To solve the aforementioned problem, various types of boneguard pads have been used in the prior art during the packaging of primal and subprimal meat cuts. A typical conventional boneguard is a wax-impregnated cloth or plastic pad which is severed from a supply roll and placed by hand over the exposed bone before the meat is inserted into the bag. These pads, which are also known as bone wrap, do not seal to the inner surface of the bag. Other boneguard structures include those such as disclosed in the U.S. Pat. No. 4,136,205 to Quattlebaum, and U.S. Pat. No. 4,267,960 to Lind. However, these boneguard structures have suffered from various limitations, including the relatively high cost of labor and materials involved in placing the boneguard pads properly onto the exposed bone ends. In addition, the boneguard pads often have not adequately protected the bags from the effects of protruding bones, so that bag punctures are still experienced during evacuation and shrinkage of the bags and during shipping of the bagged meat product.
A puncture-resistant primal meat packaging material is disclosed in U.S. Pat. No. 4,296,156 to Lustig et al. The packaging material is a four-layer film comprising a first outer layer of a heat sealable polymer, such as an ethylene-vinyl acetate copolymer, a first core layeer of an oxygen barrier material, such as a polyvinylidene chloride copolymer, a second core layer consisting essentially of an aromatic polyurethane, and a second outer layer of an ethylene-vinyl acetate copolymer.
Another puncture-resistant primal meat packaging material is disclosed in U.S. Pat. No. 4,348,437 to Lustig et al. The packaging material is a three-layer or a four-layer film. The four-layer film comprises a first layer of an ethylene-vinyl acetate copolymer, a second layer of an oxygen barrier material such as a polyvinylidene chloride copolymer or an ethylene vinyl alcohol copolymer, a third layer comprising a blend of a polyurethane and an ethylene-vinyl acetate copolymer, and a fourth layer of an ethylene vinyl acetate copolymer or blends of polypropylene copolymers with other polyolefins.
Although the four-layer puncture resistant primal meat packaging material disclosed in the two U.S. Patents to Lustig et al are improvements over earlier multilayer films, they still require the use of protective bonewrap to provide acceptable loading and shrink tunnel survival rates during the packaging of primal and subprimal meat cuts.
The present invention is based upon the discovery that four-layer films, fully described hereinbelow, comprising a first outer layer (inner surface of the bag) of a heat sealable polymer, such as an ethylene-vinyl acetate copolymer; a first core layer serving as an oxygen and moisture barrier layer; a second core layer comprising a polyurethane; and a second outer layer (outer surface of the bag) comprising linear low density polyethylene copolymer, have excellent bone puncture resistance when employed in the shrink packaging of primal and subprimal meat cuts. Accordingly, this invention provides an improved four-layer film that can be employed to fabricate bags useful for shrink packaging primal and subprimal meat cuts and similar food products.